This invention relates to an electric circuit breaker of the type that comprises a plurality of vacuum-type circuit interrupters electrically connected in parallel with each other.
A circuit breaker of this type is shown and claimed in application Ser. No. 548,847-Mitchell, filed Feb. 10, 1975, and assigned to the assignee of the present invention. In the Mitchell circuit breaker, the parallel-connected interrupters are opened substantially simultaneously during an interrupting operation, and the total current through the circuit breaker under high-current interrupting conditions is shared by the interrupters, thus reducing the interrupting duty on each interrupter. A problem encountered in this type of circuit breaker is assuring that arcs will be established concurrently in all the parallel-connected interrupters during high-current interruptions so that the total current can be shared generally equally by the interrupters. Assuming that there are two parallel-connected interrupters, a situation in which it is especially difficult to assure concurrent arcing in both interrupters is when the contacts of one interrupter part just prior to a natural current zero and the contacts of the other interrupter part just after natural current zero. In such a situation, immediately after natural current zero, all the current passes through the contacts still in engagement; and when these latter contacts part, there is a tendency for arcing to occur only between these latter contacts.
For assuring concurrent arcing in the parallel-connected interrupters during high-current interruptions, even under such circumstances, Mitchell provides a communicating passage between the interrupters through which ionized arcing products can readily pass from any inter-contact gap then arcing to one not then arcing, thereby forcing an arc-over of the intact gap. To accommodate this communicating passage, Mitchell utilizes for each of his interrupters a design that is characterized by an envelope comprising a tubular metal housing surrounding the arcing gap of the interrupter and a generally radially extending opening in this tubular metal housing leading to the communicating passage.
This approach, however, is not usable with most vacuum interrupter designs because in the usual design the envelope includes no such metal housing surrounding the arcing gap. Usually, this portion of the envelope is primarily of insulating material and therefore does not readily lend itself to the incorporation of a passageway therethrough. Providing a hole in an insulating wall and sealing an appropriate pipe or conduit thereto involves a major and expensive reconstruction of the envelope.